infer php version from composer.json

[clxmstaab]

the readme describes that rector will use the systems php version, or allows to configure a fixed version in the rector.yml

https://github.com/rectorphp/rector#exclude-paths-and-rectors

it would be great if rector could infer the php version from composer.json config->platform->php version if defined

[TomasVotruba]

Good idea